What causes laptop batteries to overheat?

http://www.howstuffworks.com/dell-battery-fire.htm

http://news.cnet.com/8301-1001_3-10241137-92.html

http://www.scienceagogo.com/news/20041109215916data_trunc_sys.shtml
http://myespow.blogspot.com/2009/02/how-to-avoid-laptop-battery-overheat.html

http://www.cbc.ca/consumer/story/2009/08/26/bc-overheating-laptop-fire-death-vancouver.html